The present invention relates to a stage apparatus for movably mounting a table on a stage, and in particular, to a support mechanism for supporting the table mounted on the stage. More particularly, the present invention relates to a stage apparatus suitable for use in an exposure apparatus, for mounting thereon a photosensitized substrate onto which a pattern formed on a reticle is transferred by exposure through a projection lens.
In a typical stage apparatus for use in an exposure apparatus and the like, a table is movably mounted on a stage at three points which do not lie on a straight line but define a triangle within which the center of gravity of the table lies. Further, the table is driven at these three points in order to provide vertical displacement and/or tilting of the table relative to the stage. The manner in which the table is movably mounted on the stage falls into two general categories: first, direct-contact actuators are used so as to support and drive the table through the direct contacts of the actuators with the table; second, noncontact actuators such as voice coil motors (VCMs) are used for the support and drive purposes.
Unfortunately, in the case where direct-contact actuators are used, stresses may be created within the table by frictional forces occurring at the contact points between the table and actuators. On the other hand, in the case where noncontact actuators such as VCMs are used, the actuators have to be continuously energized to produce vertical thrust forces for bearing the weight of the table, with the result that a considerable amount of heat is generated by the actuators and may affect the table to thereby cause harmful deformation of the table.
As one support structure usable for mounting a stage (or a table), there is used a so-called kinematic structure. Briefly, with the kinematic structure, the stage is supported by the combination of a point, a V-groove and a flat surface. This kinematic structure may be called an ideal support structure because no harmful stress is created within the stage when it is driven for vertical displacements and tilting. Nevertheless, the kinematic structure has a disadvantage in its ability to accommodate the horizontal motion of the stage and is not suitable for use in a case where the support members for the stage themselves are subject to rapid horizontal motion. For this reason, the kinematic structure has not been used for a stage apparatus used in an exposure apparatus.
FIG. 12 shows a schematic exploded perspective view of a typical stage apparatus for use in an exposure apparatus. As shown in FIG. 12, a base or surface plate 600 carries thereon a Y-stage 620 which is driven in the longitudinal direction of the surface plate 600 (or in the Y-direction) by a drive motor 630. The Y-stage 620 in turn carries thereon an X-stage 621 which is driven in the direction perpendicular to the Y-direction (or in the X-direction) by a second drive motor 631. Further, the X-stage 621 carries thereon three table support bars 640, 641 and 642 and three drive pins 720, 721 and 722 such that the table support pins and the drive pins are disposed alternately along a circle at regular angular intervals of 60 degrees. Accordingly, the three table support bars 640, 641 and 642 are disposed on three corner points of a regular triangle, respectively, while the three drive pins 720, 721 and 722 are disposed on the three corner points of another regular triangle, respectively.
A table 660 has three circular holes 670, 671 and 672 formed therein at positions corresponding to the distal (or upper) ends of the three table support bars 640, 641 and 642, respectively. The circular holes 670, 671 and 672 have a diameter slightly greater than that of the table support bars. When the table 660 is attached to the X-stage 621, the distal ends of the table support bars 640, 641 and 642 are inserted into the circular holes 670, 671 and 672, respectively, and then the table support bars 640, 641 and 642 and the table 660 are connected with each other by a ring-shaped plate spring 680 called a "flexure". More specifically, as shown in FIG. 13, the ring-shaped plate spring 680 is secured to the table support bars 640, 641 and 642 by three set screws 701, 703, 705 and to the table 660 by additional three set screws 700, 702 and 704. Thus, six set screws 700-705 in total are used for connecting them with each other. With this arrangement, the plate spring 680 tends to make the top surface of the table 660 coincident with the plane defined by the top (or upper end) surfaces of the table support bars 640, 641 and 642. In this manner, the table 660 is supported on the X-stage 621 by the plate spring 680.
With the table 660 having been thus connected to the X-stage 621, the drive pins 720, 721 and 722 are just under respective screw holes 700a, 701a and 702a formed in the table 660, as shown in FIG. 12. The drive pins 720, 721 and 722 are independently driven by respective drive units 740, 741 and 742 in the direction perpendicular to an XY-plane (or in the Z-direction), so that the table 660 is driven by the three drive pins 720, 721 and 722 at three different support points, such that the position in the Z-direction of the table 660 relative to the X-stage 621 and the tilt angles of the table 660 with respect to an XY-plane may be set to any desired position and angles within limited ranges. The plate spring 680 used here is highly rigid against any horizontal displacement of the table 660 while a low rigidity against any vertical displacement of the table 660 in order to achieve a rapid settling time when a horizontal displacement of the X-stage 621 and/or the Y-stage 620 is made for positioning the table 660.
In general, the flexure (or plate spring), mounted on the stage apparatus as described above and forming a part of an exposure apparatus, has to have rigidities meeting the following two conditions a) and b).
a) The rigidity of the flexure in the Z-direction, Kz, should be set to as small value as possible in the range that suffices the formula below for any possible displacement within the stroke of the table for the focusing and/or levelling operation: EQU M*g&gt;Kz*z
where:
M stands for the mass of the table including any parts mounted on the table; PA1 g stands for the acceleration of gravity; and PA1 z stands for the displacement of the point at which the table is driven.
b) The rigidities of the flexure in the X- and Y-directions and that in the Z.theta.-direction (which is the rotational direction about the Z-axis) are selected depending on the control characteristics of the stage, such that these rigidities should be enough to provide satisfactory settling time and control response.
The typical stage apparatus for an exposure apparatus described above uses the ring-shaped plate spring because it meets these two conditions a) and b). However, the use of the ring-shaped plate spring suffers from the following problems.
Since securing points between the table and the plate spring (flexure), and the other securing points between the table and the stage are disposed on the same circle, respectively; the plate spring tends to be deformed due to the force acting thereon in a Z-direction from the table support bars during a focusing and/or levelling operation. However, because the plate spring is highly rigid in the horizontal direction, it resists being stretched so that tensile stress occurs within the plate spring. While, the table, as shown in FIG. 13, is subjected to forces which act in the direction from a securing point between the table and plate spring to the neighboring securing point between the table and stage (these forces are represented by solid line arrows in FIG. 13). Each of the securing points on the table is subjected to two forces, respectively, tangential components of which are balanced each other, and radial components of which serves as a moment tending to deform the table (these radial components of the forces are represented by dotted line arrows in FIG. 13). As a result, the table is subjected to minute deformation (which may be coneshaped deformation, for example).
This deformation leads to the changes in distance between each of the movable mirrors 760 and 761 mounted on the stage 660 and the photosensitized substrate W held on the table 660, resulting in errors in the measured values produced by respective interferometer units 800 and 801 serving to measure the positions of the movable mirrors 760 and 761. Further, the errors cause offsets between the estimated position for a pattern printing on the substrate W as estimated from the measured positions of the X-stage 621 and the Y-stage and the actual position for the pattern printing, these offsets may affect the accuracy in alignment between the reticle (not shown) and the photosensitized substrate W as well as the accuracy in positioning the exposure shot areas on the photosensitized substrate. The deformation of the table caused according to this mechanism is typically in the order of a few nanometers, which is enough to be a problem in recent exposure apparatus.
The forces tending to cause a minute deformation of the table such as described above may be created by inertial forces acting on the table due to acceleration/deceleration of the stage, or by the deformation of the plate spring which may occur, as a result of a focusing and/or levelling operation as described above. The former factor, the inertial forces, do not occur during the exposure operation following the completion of the positioning of the stage and thus do not directly lead to any deterioration in registration, while the latter factor, the deformation of the plate spring, may occur at any time, and further, may vary depending on the displacement of the table required for the focusing and/or levelling operation, so that it is highly probable that any deformation of the plate spring causes some deterioration in registration.